In the blow molding of hollow articles from thermoplastic synthetic resin, it is known to produce tubular preforms from the synthetic resin melt, to introduce the preform as a blank into a blow mold, to close the mold onto the preform, and to expand the preform by blowing air into the preform against the blow mold and the walls thereof to form the hollow article.
Adherent waste portions, which result from closing the blow mold onto the tubular preform above and below the portion of the preform to be shaped into the hollow article, can then be removed. In this system, the synthetic resin melt can emerge from a nozzle gap of an extrusion pressing head whose extrusion or nozzle gap can be controlled in accordance with a preform blank shaping wall thickness program in which the volume of synthetic resin melt required for a preform is subdivided into a predetermined number (n) of volume segments and each of these volume segments is assigned a nozzle gap setting value such that the nozzle gap setting values form a program curve when plotted against the number (n) of volume segments.
This program curve, which is a superimposition of at least a basic gap and a profile curve, enables control or regulation of the preform extrusion or ejection amounts for each of the segments utilizing, for the control, the base gap as the control parameter. The base gap can extend uniformly in conventional systems over the n volume segments of the preform. For the production of hollow bodies of thermoplastic synthetic resin, for example, canisters, two types of machines have generally been used in practice. The first operates in a continuous system, i.e. the preform is continuously extruded. The other type of machine operates in an incremental mode known as a storage head operation. By means of a piston, a synthetic resin melt in a storage chamber is expelled from the outlet of the head in a hollow extruded preform. The expulsion is effected cyclically.
The wall thickness program controlling the nozzle gap provides regions having small gap widths and at least one region associated with a substantially greater nozzle gap width. In the production of closed containers, e.g. canisters, generally at least two regions of greatly increased nozzle gap width may be required. The program curves in these regions have function maxima. The preform segment or segments corresponding to the function maxima are generally those which are subjected in the blow mold to the greatest stretching. The more extreme the program curve in certain regions, the more significant is the quantity of material which is provided and thus corresponds to the portions of the preform to be subjected to the greatest degree of stretch. Obviously it is also essential for the proper shape of the finished article and uniformity thereof that these regions of greatest stretch and thus the most extreme portion of the program curve be positioned at the correct locations in the blow mold.
Indeed, deviations of more than .+-.1% in the height of these regions in the blow mold can result in significant detriment to the quality of the product, especially when it is in the form of large hollow bodies. Such shifting in height can result in reduced compression of compacted regions which may have to sustain high degrees of stress, nonuniform stretching of regions which ought to have had sufficient material to allow maximum stretching to occur and thus weakened zones in the product, etc. The strength and other properties of the hollow bodies which are fabricated may be sharply reduced. Furthermore, it should also be noted that it is important that the resulting hollow body have a weight which corresponds exactly to a predetermined set point weight.
Since the extrusion parameters and especially the rate at which the melt emerges from the head depend largely upon the rheological characteristics of the synthetic resin melt, the material temperature and fluctuations therein, control of the nozzle gap and, in apparatus with a continuously operating extruder, also worm speed, have required a variety of control procedures and approaches which have not always been successful.
In EP-A-0 345 474, a process is described in which monitoring of the hollow body is effected by two different weight measurements and depending upon the weight measurements the speed of the worm and/or the storage operation of the nozzle gap of the head are controlled. Preferably, the lower waste portion after removal from the blow-molded body extracted from the mold and the net weight of the hollow body after removal of the waste portions can be measured and these measured values used for the control process.
Based upon two weight measurements it is possible on the one hand to establish the position of a predetermined portion of the preform relative to the blow mold with great precision and, on the other hand, to simultaneously maintain a predetermined weight of the blow-molded hollow body.
The preform, emerging from the nozzle gap is freely formable and, by and large, is not subjected until blowing to external forces except for those which are a result of the weight of the material. Because of the visco-elastic characteristics of the melt, the distribution of the material in the preform can fluctuate and, since these visco-elastic properties themselves can fluctuate because of variations in the melt temperature, the material composition, the extrusion time, shear stresses operating on the melt and the like, a lack of precision in the material distribution can result. In addition, partly because of the interplay of swelling, shrinkage and elongation effects, there are deviations in the wall thickness of the finished product at various locations along the latter from the desired wall thicknesses.
The swelling effect is the effect which applies to the preform as it emerges from the nozzle orifice and which results form a molecular disorientation in the product within the flow channels. The shrinkage effect is the shrinkage which occurs when the preform cools after leaving the extrusion head and which is a result of a tendency of the disoriented molecules to pull together. Elongation effects are those which tend to draw the thermoplastic out as a result of the intrinsic weight of the preform.
Indeed, as the weight of the preform increases during extrusion of the preform generally downwardly, these effects result in a reduction in the diameter and wall thickness of the preform directly below the outlet of the extrusion head. The elongation tends to increase progressively from the beginning extrusion of the preform, i.e. from the lower edge of the preform and the first of the n volume segments into which the preform can be subdivided, growing greater as the number n increases.
The discharge velocity of the material at the head thus appears to increase over the course of preform extrusion. The described visco-elastic effects have a complex interdependency. With longer extrusion times and higher temperatures of the extruded mass, the elongation is greater. As the elongation tends to increase, the swelling at the outlet of the extrusion head tends to decrease. On the other hand, the swelling tends to increase upon a reduction of the nozzle gap and with an increasing speed of the extruder worm.
With longer extrusion times, the shrinkage in the lower part of the preform at the greatest distance from the nozzle gap tends to increase. A composite of these visco-elastic effects of swelling, elongation and shrinkage can be described as "sag" herein.
The form of the program curve used hitherto to control such an operation was determined by the visco-elastic properties of the synthetic resin melt for a particular temperature, mass throughput, material, nozzle gap width, extruder screw speed and optionally other operating parameters and took into consideration the sag in determining the operating point only to the extent that the sag was dependent upon these operating parameters. Deviations of the operating parameters from the values which were the basis for determining the program curve resulted in an adverse effect upon the material distribution so that, for example, there were changes in the spacings between the maxima of the material in the preform and thus variations in the product quality since there tended to be a reduction in thickness of the product at greatest stretch regions and thickenings in lesser stretch regions. The variations in material distribution could only be incompletely compensated by the prior art control process.
Consequently, even when the extruded or ejected preform had the desired weight and was positioned accurately in the blow mold, the described variations in the visco-elastic effect could give rise to the irregularities in the finished product as described, especially as to the distance between two preform segments which may include the material maxima and were the most critical for expansion or stretching of the preform. Indeed, even very small changes in this distance can lead to significant quality differences in the product.